This invention relates to an apparatus for and a method of wrapping a pipe with cementatious material, such as concrete, the pipe being rotated and advanced in relation to its longitudinal axis while a strip of wrapping material is delivered to the pipe at an angle to the longitudinal axis. Such apparatus is described, for example, in U.S. Pat. Nos. 4,544,426 (Stockman), 3,740,291 (Mallard), 4,006,049 (Gardner) and 4,333,783 (Gardner).
The strip of wrapping material generally comprises a carrier tape which carries a layer of the cementatious material, the layer being formed with notches along its edges which interlock along the wrapping seam as the material is wrapped onto the pipe so as to form an even coating. The layer of cementatious material may incorporate one or more layers of reinforcing mesh which protrude from one edge so as to overlap the wrapping seam.
In the known methods the wrapping strip is formed to the required configuration in the hopper at which the cementatious material is deposited onto the carrier tape as the carrier tape passes through the hopper. As the material leaves the hopper via a gate, it is moulded to the required thickness and configuration by a roller which carries one or more disks at one end, the disks being of larger diameter than the roller to form the notches along one edge. The notches at the other edge are typically formed by notching belts as described by Stockman.
It is important that the coating applied to the pipe be continuous and of an even thickness so that the desired weighting of the pipe is obtained and so that the coating once applied will be resistant to deterioration. Furthermore, if the material incorporates a reinforcing mesh, as it usually does, it is important that the mesh be accurately positioned in the layer and remain in position throughout the process. These things were recognized by Stockman, of course, who took important steps to mitigate the problems encountered in producing a satisfactory product. Nevertheless, in the current practice some of the problems remain. It has not been possible hitherto to achieve a consistent flow of material from the hopper to the pipe, or to obtain a layer of the cementatious material of uniform density throughout its cross section. The problem of obtaining a satisfactory wrapping strip becomes more acute when the strip incorporates multiple reinforcing meshes.
One reason for this is that the cementatious material in the hopper must work its way down through the reinforcing mesh to be embedded in it, thus weighing down on the mesh and displacing it from the selected level. Attempts to combat the problem by providing extended guides for the mesh and/or by applying more tension to it have failed to solve the problem of mesh displacement and/or distortion. Another reason is that when the notched strip of cementatious material is moulded by the roller and notching disk combination, the different peripheral speeds of the roller and the disk, acting on different parts of the strip, result in different compressive forces and nonuniformity of the material. This also results in forcing the reinforcing mesh out of position.